This invention relates to an apparatus for tightening and releasing a pressure vessel clamping nut, i.e. one of several nuts which act to maintain a preestablished pressure-tight sealing of for example a reactor pressure vessel, the bolts for which nuts are held and pretensioned in a flange of the pressure vessel.
It is known for such apparatus to comprise an electric motor connected to a gearing adapted to co-operate with a teeth on the nut. The use of an electric motor for the drive requires less outlay than that of a hydraulic motor. Also, for a reactor pressure vessel, there is no danger of radioactive contamination of any oil escaping from a leak.
With a known apparatus the nut can be tightened into its clamping position on the pretensioned bolt, but a difficulty arises when the nut is to be released from its clamping position. This is that the electric motor, when tightening the nut, cannot be switch off so precisely that for subsequently releasing the nut it is able to provide a starting torque greater than the tightening torque of the nut. The same difficulty is encountered where there is interposed between the electric motor and the nut a coupling, i.e. clutch, which is operated by control pulses.
The use of safety couplings in conjunction with a switching-off of the motor in dependence upon the increase in current which occurs in the event of increase in the tightening torque has also proved unsuccessful, because an accurate monitoring of the tightening torque is not possible.